var isSymmetric = function(root) {
    if(root==null)return true
    return duicheng(root.left,root,right)
}
function duicheng(p,q){
    if(p==null||q==null)return p==q
    return p.val==q.val&&duicheng(p.right,q.left)&&duicheng(p.left,q.right)
}